The present invention is directed to an apparatus for managing system interrupt operations in a computing system including a processing unit and a plurality of peripheral devices. Interrupt signals are generated by peripheral devices in a computing system to notify a processing unit that access to the processing unit is required in order to effect an operation such as a memory read, a memory write, a mathematical calculation, or the like.
Some computing systems employ a simple input to the processing unit (an "interrupt line") to notify the processing unit that a given peripheral device desires access to the processing unit. Other systems employ what is known as a "system management interrupt" (SMI) which provides a line generally connected as a wired-OR line among the various peripheral devices and the processing unit. The wired-OR line is, in effect, an input-output (IO) transmission line with respect to the processing unit in that it provides the interrupt signals generated by the various peripheral devices as an input to the processing unit. The processing unit introduces an acknowledge signal as an output on the transmission line to notify the various peripheral devices affected that the processing unit is in an interrupt mode and, by another means, signals the peripheral units to terminate their transmission of interrupt signals on the transmission line. Thus, the transmission line of an SMI system may also be characterized as an interrupt/modal line.
The SMI transmission line is also an IO line with respect to peripheral devices with which it is connected. That is, an interrupt signal generated by a peripheral device constitutes an output from the peripheral device, and the acknowledge signal introduced to the transmission line by the processing unit is an input to the peripheral device.
As a wired-OR line, an SMI system requires a pull-up resistor to hold the transmission line HIGH (INACTIVE) when it is not driven. The resistance of the pull-up resistor dissipates power when a driver (either in a peripheral device or in the processing unit) is driving the transmission line. Negligible power dissipation occurs during periods when the transmission line is not driven. The choice for resistance value of a pull-up resistor involves a design trade-off because a high resistance will effect less power dissipation than will a low resistance during periods when the transmission line is driven. However, such a higher resistor value participates with the inherent capacity of the computing system and its various interconnections to generate an RC time constant which determines the rate of decay of the signals carried on the transmission line when the line transitions from a driven state to a non-driven state. Thus, a high value of a pull-up resistor will result in a longer decay interval during which the driven transmission line decays to its non-driven state. One can shorten the decay interval by reducing the value of the pull-up resistor; however, this results in greater power dissipation during times when the transmission line is driven.
Therefore, with existing apparatuses, in applications where power is desirably conserved (e.g., battery-operated computers) but where speed of operation is also a desirable factor, a designer must carefully choose the value of the pull-up resistor on the transmission line for a system management interrupt apparatus and must, necessarily, accept some trade-offs between system response time and power dissipation.